(1) Field of the Invention
The present invention relates to a method and an apparatus for controlling supercharge pressure for vehicles having a turbocharger and an internal combustion engine supercharged by the turbocharger.
(2) Description of the Prior Art
It has been widely practiced heretofore that intake air is supercharged by a turbocharger in order to improve the output power of the internal combustion engine.
The turbocharger comprises an exhaust gas turbine driven by the exhaust energy of the engine, and an intake air compressor capable of being rotated by the exhaust gas turbine for supercharging the intake air. The exhaust gas turbine is rotated at high speed when the exhaust gas flow rate of the engine increases and it can drive the compressor efficiently. However, in a low load operation zone of the engine having a small exhaust gas flow rate, it is inevitable that the supercharge efficiency of the exhaust gas turbine lowers.
For the purpose of preventing the number of rotations of the exhaust gas turbine from lowering and efficiently supercharging the turbine up to a relatively small engine load zone, a supercharge pressure apparatus has been already proposed in which a control valve is provided at the inlet of the exhaust gas turbine for controlling the exhaust gas flow speed and the opening of the control valve is controlled in accordance with the operating conditions of the engine, thereby reducing the opening of the control valve in a low load operation zone having less exhaust gas flow, so as to increase the exhaust gas flow and meet the purpose (see, for instance, U.S. Pat. No. 2 944 786).
In this kind of the supercharge pressure control apparatus according to the prior art, however, it was difficult to control a control valve accurately in accordance with the operating conditions of the engine. Namely, when the control valve is excessively closed, the supercharge pressure is increased more than a target supercharge pressure, while if the control valve is excessively opened, the supercharge pressure is lowered.
In addition, if the correction or compensation for deviation is delayed in the direction of decrease in the supercharge pressure when the supercharge pressure is increased, damages due to a knocking condition of the engine and an excessive load condition wear tends to be invited. Furthermore, when the valve control is suddenly performed, an overshoot condition in the supercharge pressure occurs and a stable control for the supercharge pressure control tends to be difficult. Moreover, in the supercharge control according to the prior art, the target value of supercharge pressure to be controlled was always maintained constant regardless of the operating conditions of the engine. As a result, it was difficult to realize an optimum supercharged condition required for the engine. It also resulted in the problem that a sufficient improvement in the output characteristics and fuel consumption rate could not necessarily be achieved.